How to Highlight, Underline, and Comment on a PDF Without Uploading It

Published 14 July 2026 · 5 min read

Reviewing a PDF almost always means marking it up somehow — highlighting the part that matters, underlining a clause, sketching a circle around a figure, or leaving a comment for whoever reads it next. Most of the tools built for this, from full desktop readers to browser-based markup sites, want the file uploaded to a server first, even for something as simple as adding a highlight to one paragraph.

Markup and redaction are opposites, not variations on the same thing

It's worth being precise about this, since the two get lumped together casually. Redaction exists to permanently destroy content — once something is redacted properly, it's actually gone from the file. Markup is built to do the exact opposite: highlight, underline, and comment annotations sit on top of a page without touching the original text or graphics underneath at all. Anyone opening the file can still select and copy exactly what was there before, and can hide, edit, or delete the markup itself in any real PDF reader — the same as clearing a comment in Acrobat.

Why "real annotation" is a meaningful distinction, not just a technicality

A lot of markup tools take a shortcut: they flatten the page into an image and draw directly onto the pixels, which looks identical at a glance but produces a fundamentally different result. A flattened highlight can't be toggled off, can't be edited later, and permanently bakes itself into what used to be sharp, selectable text. A genuine PDF annotation — the same underlying object type Adobe Acrobat, Preview, and every other serious PDF reader use for their own highlight and comment tools — stays a separate, removable layer, exactly as markup is supposed to work.

What FormatDog's Annotate & Markup tool actually builds

Annotate & Markup PDF supports four real annotation types: a translucent highlight, a colored underline, freehand ink strokes for sketching or circling something, and sticky-note style comments placed with a single tap. Each one includes its own visual appearance built directly into the file, rather than counting on whichever app opens it next to guess how it should look — which is what makes markup show up correctly across different PDF readers, not just inside the tool that created it.

Checking any of this yourself

Same trick as always: open developer tools (F12), click the Network tab, mark up a PDF, and watch for outgoing requests. Nothing resembling your document's content should appear — the reading, marking, and rebuilding all happen on your own device. And to confirm the markup is genuinely removable rather than flattened, open the result in any other PDF reader and try toggling a highlight or comment off — a real annotation lets you; a flattened image never would.